In a known technique that is utilized for air-fuel ratio feedback control or for evaluating the oxygen storage capacity of a catalyst, an oxygen sensor is arranged downstream of a catalyst that is mounted in an exhaust passage of a vehicle to detect changes in the exhaust gas air-fuel ratio downstream of the catalyst. Thus, the oxygen sensor that is arranged downstream of the catalyst takes low-concentration exhaust gas that has been purified by the catalyst as a detection object. Accordingly, it is desirable that the oxygen sensor can detect a change in the air-fuel ratio of low-concentration gas with high sensitivity.
For example, Patent Literature 1 discloses a technique that improves the sensitivity of a limiting-current-type sensor to gas in a low-concentration range. According to the technology described in Patent Literature 1, when the detected gas is in a low-concentration range, a voltage whose polarity is opposite to the polarity of a voltage applied at a time of normal measurement is applied to electrodes of the sensor, and a current value is detected. According to the invention described in Patent Literature 1, it is thereby possible to detect the concentration of gas in a low-concentration range with high sensitivity.